1. Field of the Invention
The present invention relates to a fixing unit and roller member for use in an image forming apparatus using, for example, an electrophotographic method.
2. Description of the Related Art
In an image forming apparatus such as a copy machine and a printer which use an electrophotographic method, a photoreceptor that is drum-shaped for example (photoreceptor drum) is uniformly charged, and this photoreceptor drum is scan exposed by light controlled based on image information, thus forming an electrostatic latent image on the photoreceptor drum. And, this electrostatic latent image is converted into a visible image (toner image) using toner, and after directly transferred to recording paper from on the photoreceptor drum, or after once primary-transferred to an intermediate transfer body and then secondary-transferred to the recording paper from the intermediate transfer body, this toner image is fixed to the recording paper by a fixing unit.
The fixing unit for use in such an image forming apparatus includes, for example, a fixing roller and a pressure roller. The fixing roller, having a heating source disposed within a cylindrical core metal, is formed by laminating on the core metal a heat-resistant elastic layer and a release layer. The pressure roller, disposed in pressure contact with this fixing roller, is formed by laminating on the core metal a heat-resistant elastic layer and a release layer of heat-resistant resin film or heat-resistant rubber film. And, recording paper bearing an unfixed toner image is passed between the fixing roller and the pressure roller to heat and pressurize the unfixed toner image, thereby fixing a toner image to the recording paper. Such a fixing unit, which is of so-called roller-nip method, is widely used.
In the meantime, for an increase in speed in the fixing unit of roller-nip method, to supply a sufficient amount of heat to the toner and the recording paper, it is necessary to increase a nip width in proportion to a fixing speed. As the method of increasing the nip width, there are the method of increasing a load between the fixing roller and the pressure roller, the method of increasing the thickness of an elastic body, and further the method of increasing a roller diameter.
However, in the method of increasing the load and the method of increasing the thickness of the elastic body, roller deflection makes the shape of the nip width nonuniform along a roller axis, thereby causing a problem which may affect image quality, such as the occurrence of variations in fixability or cockles in paper. Besides, in the method of increasing the roller diameter, there is the problem of involving an increase in size of the apparatus and of lengthening a period of time (warm-up time) required to increase a roller temperature from a room temperature to a fixable temperature.
Thereupon, to solve these problems to realize a fixing unit responsive to an increase in speed of the image forming apparatus, the present applicant provides technology for a fixing unit of the following configuration (e.g., see Japanese Patent No. 3084692). That is, the fixing unit includes a fixing roller having an elastic body coated on a surface thereof and an endless belt stretched over plural support rollers, wherein the endless belt is wrapped a predetermined angle region around the fixing roller so as to form a nip region between the endless belt and the fixing roller. At the same time, disposed in an exit portion (the most downstream portion) of the nip region is a pressure roller that locally applies thereto higher pressure than any other portion of the nip region.
In such a fixing unit described in the Japanese Patent No. 3084692, the endless belt stretched over the plural rollers is brought into contact with the fixing roller, thereby forming a nip portion (which is also called a “belt-nip portion”). By adopting such a configuration (which is also called a “belt-nip method”), the width of the belt-nip portion formed by the fixing roller and the endless belt can be easily made larger than the width of the related-art roller-nip portion between the fixing roller and the pressure roller. This enables a response to the increase in speed and also facilitates a reduction in size of the apparatus.
Particularly, in the fixing unit of belt-nip method, the heat capacity of the endless belt brought into pressure contact with the fixing roller is low, so that heat transferred from the fixing roller is also difficult to diffuse. Consequently, even when the rotation of the fixing roller is started, the amount of heat drawn from the fixing roller to the endless belt side is comparatively small, thus increasing efficiency in the use of heat for fusing toner. Therefore, this fixing unit also has the advantage of being capable of improving toner fixability.
In the meantime, the fixing unit of belt-nip method as described in the aforesaid Japanese Patent No. 3084692 is configured such that one of the rollers having the endless belt stretched thereover presses against the fixing roller (fixing member) in the most downstream portion of the nip portion formed by the fixing roller and the endless belt. Thereby, such a roller is made to function as a pressure roller (pressure member) and thus is set to locally apply, to the most downstream portion, higher pressure than any other portion of the nip portion. By adopting such a configuration, it follows that the pressure roller applies local pressure to a toner image being heated and fused in the nip portion. Consequently, the fixability of the toner image passed through the nip portion is improved, and particularly the surface of the toner image before solidified is smoothed, thus imparting proper gloss to the toner image.
However, the pressure roller, which presses against the fixing roller in the most downstream portion of the nip portion, has the endless belt stretched therearound on an axially lateral peripheral surface thereof. Consequently, to set such a pressure roller to press toward the fixing roller side, it is necessary to adopt the configuration of pressing the fixing roller from both end portions of the pressure roller. However, in the configuration in which the pressure roller thus presses the fixing roller from the both end portions thereof, arcuate deflection occurs in the pressure roller (even in the fixing roller as the case may be). Therefore, there is the structural problem in which the pressure between the pressure roller and the fixing roller decreases toward an axially central portion of the pressure roller, so that nonuniformity in axial pressure is likely to occur.
Consequently, a declining trend of fixability occurs across a central portion of the recording paper, thus causing the disadvantage of a reduction in image quality. Besides, particularly, there occur so-called gloss variations in which the gloss level of the toner image decreases toward the central portion of the recording paper. Thus, there is the disadvantage that the reduction in image quality is conspicuous in a solid image such as a photographic image.